The present invention relates to improved well completion methods utilizing foamed cements and foamed well treatment fluids. Additionally, the present invention provides improved foamed cement and foamed well treatment fluids and methods for preparing and using the same.
Foamed hydraulic cement compositions are often utilized in cementing subterranean zones penetrated by well bores. For example, foamed cement compositions are used in primary well cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. A primary cementing operation normally pumps a cement composition into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. Thus, the cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. In this manner, the cement precludes the undesirable migration of fluids between zones or formations penetrated by the well bore.
The geological nature of the formation will dictate the type of cement composition appropriate for cementing operations. In order to prevent excessive hydrostatic pressure from unintentionally fracturing the formation, certain formations will require lightweight cement. To achieve the lower density of a lightweight cement, a foamed cement composition contains compressed gas which improves the ability of the cement composition to maintain the pressure necessary to prevent the flow of formation fluid into and through the cement composition during the transition time, i.e., the time during which the cement composition changes from a true fluid to a hard set mass. Foamed cement compositions are also advantageous because they have low fluid loss properties due to the two phases of the system. Therefore, the industry recognizes the benefits of and frequently uses foamed cement and other foamed fluids in sealing casing in subterranean wells.
Although similar to a conventional cementing operation, the process of foaming a cement or wellbore fluid requires additional equipment not commonly associated with conventional cementing operations. Typically, a hydraulic cement composition is foamed by combining a mixture of foaming and foam stabilizing surfactants with the cement composition on the surface. Subsequently, as the cement composition is pumped into the well bore a gas, typically nitrogen, is injected into the cement composition. This process allows the cement composition to have a downhole gas concentration, or foamed quality, of from about 20% to about 80%, by volume of the cement composition depending on the downhole pressure and temperature. This process requires the presence of nitrogen (liquid and/or gas) storage, vaporization, and pumping equipment at the well site. Normally equipment of this nature is not used at well sites and may be unavailable or difficult to transport to remote well sites. Clearly, the use of nitrogen for foaming cement and wellbore fluids complicates and adds to the overall burden of foam cementing a wellbore.
In the designing of the foam cement job, prediction software is utilized to estimate the amount of pressure, if any, required to control the expansion of the foamed fluids. If excessive expansion is encountered, the gas bubbles will become relatively large and coalesce which will increase the permeability of the set cement. To accomplish primary cementing with foam cement, it is desirable to have the wellhead equipped with annular pressure-containing devices. When pressure-containing devices are not feasible, the preferred practice runs an unfoamed cement cap ahead of the foamed cement. The unfoamed “cap” interval should be tailored for each specific job. Typically, an interval of 200-feet is considered the minimum length for the unfoamed cap.
For safety and clean-up ease, the return relief lines are typically staked and chained to exit in an acceptable waste area, such as a sump pit. Foamed cement under pressure will expand in volume prolifically if released at atmospheric pressure. This foam is more of a nuisance than a problem since it develops little strength and can be washed away with a pressure hose.
In view of the problems inherent to foaming with nitrogen, it would be desirable to provide improved foamed cements and foamed well treatment fluids that do not require nitrogen to generate the foamed cement slurry. Further, considering the inherent risks associated with oxygen in the downhole environment, it would be desirable to provide a foamed cement wherein the foaming gas is air having a reduced oxygen content.